This invention relates to a device for measuring active and/or reactive component of current or power in an AC circuit.
It is commonly known that measurement of active power and reactive power in an AC power supply system is necessary for the rational control or management of the AC power supply system.
Measuring instruments of various types including the electrodynamometer type, inductive type and thermocouple type have heretofore been widely used for the measurement of active power, and for special purposes, electronic multipliers have been used to obtain the product of instantaneous values of AC voltage and current.
However, due to the fact that the prior art multiplier of the electromagnetic type such as the electrodynamometer type or inductive type is based on such a structural principle that its output is produced by torque, such output is not suitable for application as an input signal to an electronic control circuit or like circuit. Especially, the measuring instrument of the inductive type has such a defect that it is accompanied by a large error at a low power factor.
The measuring instrument of the thermocouple type can be used as a detector for control purposes as its output is the electromotive force produced by the thermocouple. However, this thermocouple type is also defective in that it can only provide a small electromotive force which necessitates additional provision of a large-gain amplifier.
The measuring instrument using the electronic multiplier for multiplying the instantaneous voltage and current values is suitble for control purposes. However, this electronic multiplier is generally quite expensive. An IC multiplier may be used for the purpose of multiplication of the above kind. However, this IC multiplier is generally based on such a structural principle as to utilize the ability of a semiconductor circuit to perform the exponential function and is therefore defective in that it lacks the stability of offset.
Measurement of reactive power has heretofore been attained by shifting the phase of an input to the active power measuring instrument by .pi./2 using an inductance element or capacitance element. In the case of measurement of reactive power in a balanced three-phase circuit, phase current in one phase and line-to-line voltage in the remaining two phases are applied into an ordinary wattmeter. Thus, application of this principle of measurement to an unbalanced circuit is difficult to be put into practice.
A measuring instrument using an electronic circuit for the computation of ##EQU1## has also been proposed. However, this measuring instrument is defective in that not only addition of an expensive circuit to the active power measuring instrument is required, but also its output is not so precise.